The present invention relates to an optical detector, operating by direct reflection, for detecting presence or absence of an object.
Known detectors, also called proximity detectors, operate either by transmission or by reflection, and are called upon to work equally in one mode or the other.
Transmission detectors perceive the interruption of a light beam by an object to be detected. They necessarily comprise a unit for the transmission of a beam of light rays and a unit for the detection of the beam of light. Such apparatus has a limited application if it is not possible to dispose the two units in a straight line which only the object to be detected can interrupt.
Known reflection-type optical detectors are of two kinds. In both cases, the transmission of a beam of light rays and the detection of the same beam after reflection or diffusion take place by means of members in the same unit. In the first kind, the reflection-type detector can detect the light reflected by an object to be detected and in the second kind they are sensitive to the interruption of the beam reflected by a specially provided reflector. Detectors of this second indirect reflection type have the same disadvantages as those operating by transmission in regard to the interruption of the beam by any object.
Reflection-type optical detectors sensitive to light rays directly reflected by an object to be detected are constructed in various forms. The transmitted beam can be divergent, parallel or convergent. The reflected rays are collected by a second optical system, or by the same optical system as the transmitted rays, in which case they are thereafter separated by a semi-transparent mirror or an apertured mirror for directing them onto a photo-detector.
All these detectors are very severely limited in their performances in the detection of small objects from a great distance. The detector heads must be brought to within a few millimeters of the object to be detected, which is often impossible for reasons of space.
A solution already proposed for overcoming this difficulty is to make use of optical fibres for bringing the light rays into proximity to the object and collecting them. This solution also has its limits. The constructions having the best performance include those which utilise the transmission of a convergent beam and the reception of the rays reflected along a separate optical path. The directions of illumination and observation are at a certain angle. However, the further the zone of detection is from the detector, the larger the detector becomes.